In some mods, you can fit two 18350's when the mod is configured for an 18650. Or two 18650 batteries if the mod is designed for this. This boosts the voltage output from 3.6V to 7.2V. Yay! BUT, you should probably never do this. Unless you are familiar with matching batteries, you are only endangering yourself and others. You may think, "Well, shucks, laptops have multiple batteries in them, why can't my e-cig?"
Laptop batteries are specifically designed for stacking and are paired for use at the factory. When using un-matched batteries, they will discharge and provide current at different rates. This means one battery will continuously take more of the stress than the other. You may not notice any trouble at the start, but as the batteries age with use the problem will become more severe. Push the batteries a bit further and now you've entered the realm of thermal runaway. Downside being that one battery entering thermal runaway will push the other battery into thermal runaway. Now you have two small, flaming explosives only inches from your face.
For those who choose to stack batteries in series anyway, its a little safer to match, pair, or "marry" the batteries so that the above situation is decreased. This implies buying two new, exact same batteries with the same specs, marking them as a pair, and alternating the order that they are placed in the mod each and every time. Stacking batteries is considered to be more risky for a mechanical mod than a regulated mod.
Serial vs Parallel Dual Battery Mods
Some mods are designed to use two 18650 batteries simultaneously. Series (batteries stacked end to end) will double the voltage output, but the capacity (mah) and amps will stay the same as using only one battery alone. Parallel (side by side) batteries will double the amp and mah capacity. Serial and Parallel Battery Configurations
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